1. Field of the Invention
The present invention relates to a vehicle automatic transmission, and more particularly to a vehicle automatic transmission having a hydraulic torque converter and a multiple-stage transmission gear mechanism connected with the torque converter. More specifically, the present invention pertains to a control of a vehicle automatic transmission of the aforementioned type.
2. Description of Prior Art
In a vehicle having an automatic transmission comprised of a torque converter and a multiple-stage transmission gear mechanism, a shift lever is provided for selecting one of N-range (neutral), D-range, 2-range, 1-range, R-range (reverse) and P-position (parking). It has been experienced in this type of vehicle that a shifting shock is produced when the shift lever is moved from the N-range to a running range such as the D-range, 2-range and 1-range. This type of shifting shock is sometimes referred to as the "N-D shock" and caused by the fact that a power transmitting gear train is established in the transmission gear mechanism through engagements of appropriate friction members when the shift lever is actuated to make it possible to transmit the engine output power through the gear mechanism to the wheels of the vehicle. It should be noted that in this instance the transmission gear mechanism assumes a first gear stage wherein the engine output torque is multiplied with a largest ratio so that the driving wheels are suddenly subjected to a large driving torque. It should further be noted that in the first gear stage the hydraulic pressure for engaging the friction members is higher than in other gear stages because the friction members are subjected to higher reaction forces than in other gear stages. Thus, the engagements of the friction members with such high pressure can produce a noticeable shift shock.
In order to solve the problem, the U.S. Pat. No. 4,222,292 proposes to make a higher gear stage at first engage when the shift lever is actuated from the N-range to one of the running ranges and then make the first gear stage engage. Due to the fact that the driving torque in the higher gear stage is smaller than in the first gear stage and the hydraulic pressure for making engagement of the friction members for the higher gear stage is smaller than that for the first gear stage, the shift shock can be moderated. It will also be understood that the shift shock is not so significant when the transmission gear mechanism is shifted from a higher gear stage to the first gear stage as compared with a shifting from the neutral to the first gear stage.
It should be noted however that the time for maintaining the gear machanism in the higher stage is important and should be strictly determined because, if the time is too short, the gear mechanism is shifted to the first stage before the higher stage is completely established possibly resulting in a loss of shock reduction effect. If the time is too long, the gear mechanism may not be in the first stage when the driver intends to start the vehicle. It is therefore desirable that the time be strictly determined so that the higher gear stage is completed without fail but the gear mechanism is immediately shifted down to the first stage after the higher stage is completed.
According to the proposal by the aforementioned U.S. patent, it is suggested to provide a fixed time for bringing the gear mechanism into the higher gear stage. It will be necessary to determine the fixed time to a value substantially equal to the time wherein the engagements of the friction members for the higher gear stage can just be completed, however, this is very difficult in actual practice because the time required for the engagements of the friction members is not always constant but changes depending on possible manufacturing tolerances of parts or on possible dimensional changes after manufacture. Further, the time may also change depending on the viscosity of working fluid which may change depending on the temperature. The time is also a function of the pressure of the working fluid which changes in accordance with the engine output. Thus, the control as proposed by the U.S. patent cannot provide a satisfactory result.
A further problem encountered in a vehicle having an automatic transmission is that, when the vehicle is stopped with the shift lever is positioned in one of the running ranges so that the transmission gear mechanism is in the first stage, the engine idling vibrations are transmitted through the transmission to the vehicle body. It is possible to solve the problem by shifting the gear mechanism to a higher gear stage, however, if the higher stage is maintained until the time when the vehicle is to be started, it becomes difficult to start the vehicle smoothly. If the gear mechanism is shifted down to the first stage sufficiently earlier than the time for the vehicle start, there will remain the problem of the idling vibrations.